Radio communication networks were originally developed primarily to provide voice services over circuit-switched networks. The introduction of packet-switched bearers in, for example, the so-called 2.5G and 3G networks enabled network operators to provide data services as well as voice services. Eventually, network architectures will likely evolve toward all Internet Protocol (IP) networks which provide both voice and data services. However, network operators have a substantial investment in existing infrastructures and would, therefore, typically prefer to migrate gradually to all IP network architectures in order to allow them to extract sufficient value from their investment in existing infrastructures. Also to provide the capabilities needed to support next generation radio communication applications, while at the same time using legacy infrastructure, network operators could deploy hybrid networks wherein a next generation radio communication system is overlaid onto an existing circuit-switched or packet-switched network as a first step in the transition to an all IP-based network. Alternatively, a radio communication system can evolve from one generation to the next while still providing backward compatibility for legacy equipment.
One example of such an evolved network is based upon the Universal Mobile Telephone System (UMTS) which is an existing third generation (3G) radio communication system that is evolving into High Speed Packet Access (HSPA) technology. Yet another alternative is the introduction of a new air interface technology within the UMTS framework, e.g., the so-called Long Term Evolution (LTE) technology. Target performance goals for LTE systems include, for example, support for 200 active calls per 5 MHz cell and sub 5 ms latency for small IP packets. Each new generation, or partial generation, of mobile communication systems add complexity and abilities to mobile communication systems and this can be expected to continue with either enhancements to proposed systems or completely new systems in the future.
LTE uses orthogonal frequency division multiplexing (OFDM) in the downlink and discrete Fourier transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station (typically referred to as an eNodeB in LTE) transmits control information indicating to which terminals and on which resource blocks the data is transmitted during the current downlink subframe. This control signalling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink system with 3 OFDM symbols as the control region is illustrated in FIG. 3.
The interest in deploying low-power nodes (such as pico base stations, home eNodeBs, relays, remote radio heads, etc.) for enhancing the macro network performance in terms of the network coverage, capacity and service experience of individual users has been constantly increasing over the last few years. At the same time, there has been realized a need for enhanced interference management techniques to address the arising interference issues caused, for example, by a significant transmit power variation among different cells and cell association techniques developed earlier for more uniform networks.
In 3GPP, heterogeneous network networks have been defined as networks where low-power (e.g., pico) nodes of different transmit powers are placed throughout a macro-cell layout, implying also non-uniform traffic distribution. Such networks are, for example, effective for capacity extension in certain areas, so-called traffic hotspots, i.e. small geographical areas with a higher user density and/or higher traffic intensity where installation of pico nodes can be considered to enhance performance. Heterogeneous networks may also be viewed as a way of increasing the density of networks to adapt for the traffic needs and the environment. However, heterogeneous networks bring also challenges for which the network has to be prepared to ensure efficient network operation and superior user experience. Some challenges are related to the increased interference in the attempt to increase small cells associated with low-power nodes (i.e., cell range expansion) and other challenges are related to potentially high interference in uplink due to a mix of large and small cells.
According to 3GPP, heterogeneous networks comprise networks where low power nodes are placed throughout a macro-cell layout. The interference characteristics in a heterogeneous network can be significantly different than in a homogeneous network, in downlink or uplink or both.
FIG. 4 illustrates a few situations that may occur in a heterogeneous network. In FIG. 4, user equipments (UEs), which are located within a macro-cell 10 may be served by a high power base station 12. UEs within cells 15a, 15b, 15c and 15d may be served by low power (e.g., pico) base stations 17a, 17b, 17c, and 17d, respectively. The cells 15a, 15b, 15c, and 15d are smaller than the macro-cell 10 and overlap, at least partially, the macro-cell 10.
A UE 18a, which is located both in cell 10 and in cell 15a and is served by the base station 12, suffers interference from the base station 17a. A UE 18b, which is located in an area where the cell 15b overlaps the macro-cell 10 and is served by the base station 12, can cause severe interference towards base station 17b. A UE 18c, which is located an area where the cell 15c overlaps the macro-cell 10 and is served by the base station 17c, suffers interference from base station 17b. A UE 18d, which is located an area where the cell 15d overlaps the macro-cell 10 and is served by the base station 17d, suffers interference from the base station 12.
Another challenging interference scenario occurs with so-called cell range expansion, when the traditional downlink cell assignment rule diverges from the RSRP (i.e., Reference Signal Reference Power) based approach, e.g. towards a pathloss-based approach or a pathgain-based approach adopted for cells with a transmit power lower than neighbor cells. The cell range expansion is illustrated in FIG. 5. A high power (macro) base station 20 is capable of serving UEs within a cell having a radius 21 (i.e., the small-dashed line), and a low power (pico) base station 22 is conventionally capable of serving UEs within a cell having a radius 23 (i.e., the large-dashed line). When the cell range of the cell served by the base station 22 is expanded according to a Δ parameter, a wireless device 25 may potentially be within the range served by the base station 22 and it may be served by the base station 22 instead of being served by the base station 20 when cell selection/reselection occurs. The cell range expansion, indicated by the Δ parameter between points A and B in FIG. 5, is limited by the DL (downlink) performance since UL (uplink) performance typically improves when the cell sizes of neighbor cells become more balanced.
In wireless networks, maintaining a good signal quality is a requirement in order to ensure reliable and high-bitrate transmissions as well as robust control channel performance. The signal quality is determined by the received signal strength and its relation to the total interference and noise received by the receiver. A good network plan which, among other things, includes cell planning, is a prerequisite for successful network operation, but it is static. For more efficient radio resource utilization, such a network plan has to be complemented at least by semi-static and dynamic radio resource management mechanisms, which are also intended to facilitate interference management, and by deploying more advanced antenna technologies and algorithms.
One way to handle interference is, for example, to adopt more advanced transceiver technologies, e.g., by implementing interference suppression or interference cancellation mechanisms in receivers. Another way, which may or may not be complementary to the former, is to design efficient interference coordination algorithms and transmission schemes in the network. The coordination may be realized in a static, a semi-static or a dynamic fashion. The static or semi-static schemes may rely on reserving time-frequency resources (e.g., a part of the bandwidth and/or time instances) that are orthogonal for strongly interfering transmissions. Such interference coordination may be implemented for all channels or for specific channels (e.g., data channels or control channels) or signals. A dynamic coordination may be implemented, e.g., by scheduling.
Enhanced inter-cell interference coordination (eICIC) mechanisms have been developed specifically for heterogeneous networks. Some of these (now standardized) mechanisms are designed to ensure that the UE performs at least some measurements, such as, measurements for radio resource management (RRM), measurements for radio link monitoring (RLM), and measurements for channel state information (CSI), in low-interference subframes. These mechanisms involve configuring patterns of low-interference subframes at transmitting nodes and configuring measurement patterns for UEs.
Two types of patterns have been defined for eICIC to enable restricted measurements in DL (downlink): restricted measurement patterns, which are configured by a network node and signaled to the UE, and transmission patterns, also known as Almost Blank Subframe (ABS) patterns, which are configured by a network node, describe the transmission activity of a radio node, and may be exchanged between the radio nodes.
To enable restricted measurements for RRM, RLM, CSI as well as for demodulation, the UE may receive (via Radio Resource Controller) UE-specific signaling the following set of patterns (as described in TS 36.331 v10.1):                Pattern 1: A single RRM/RLM measurement resource restriction for the serving cell;        Pattern 2: One RRM measurement resource restriction for neighbor cells (up to 32 cells) per frequency (currently only for the serving frequency);        Pattern 3: Resource restriction for CSI measurement of the serving cell with 2 subframe subsets configured per UE. A pattern is a bit string indicating restricted and unrestricted subframes (i.e., subframes of a first type and subframes of a second type) characterized by a length and a periodicity, which are different for FDD (frequency-division duplex) and TDD (time-division duplex), e.g., 40 subframes for FDD and 20, 60 or 70 subframes for TDD. Restricted measurement subframes are configured to allow the UE to perform measurements in subframes with improved interference conditions, which may be implemented by configuring Almost Blank Subframe (ABS) patterns at eNodeBs. The current TS 36.331 v10.1 defines only intra-frequency restricted measurement patterns (also known as measurement resource restriction patterns), although similar patterns may also be defined for UE inter-frequency measurements, such as, inter-frequency cell search, reference signal received power (RSRP), reference signal received quality (RSRQ), positioning measurements, etc. Thus, the measurement pattern can be configured for measuring inter-frequency cells on each frequency carrier. Similarly the measurement patterns can also be used for performing inter-RAT E-UTRAN measurements. In this case, the cell on the serving RAT (e.g. UTRAN, GERAN, CDMA2000, HRPD etc) will configure the pattern enabling UE to perform inter-RAT E-UTRAN measurements (e.g., inter-RAT E-UTRAN cell search, RSRP, RSRQ, positioning measurements, etc.).        
The restricted measurement patterns are provided to the UE via dedicated signaling and thus apply only for UEs in CONNECTED mode. For UEs in IDLE mode, similar patterns may be provided via broadcast signaling.
An ABS pattern indicates subframes when the eNodeB restricts its transmissions (e.g., does not schedule or transmits at a lower power). The subframes with restricted transmissions are referred to as ABS subframes. Currently, eNodeBs can suppress data transmissions in ABS subframes but the ABS subframes cannot be fully blank—at least some of the control channels and physical signals are still transmitted. Examples of control channels that are transmitted in ABS subframes even when no data is transmitted are PBCH (Physical Broadcast Channel) and PHICH (Physical Harq Indicator Channel). Examples of physical signals that have to be transmitted, regardless of whether the subframes are ABS or not, are cell-specific reference signals (CRS) and synchronization signals (PSS and SSS). Positioning reference signals (PRS) may also be transmitted in ABS subframes.
If an MBSFN (Multi-Media Broadcast over a Single Frequency Network) subframe coincides with an ABS, the subframe is also considered as ABS. CRS are not transmitted in MBSFN subframes, except for the first symbol, which allows for avoiding CRS interference from an aggressor cell to the data region of a measured cell.
ABS patterns may be exchanged between eNodeBs, e.g., via X2 interface, but information about these patterns is currently not transmitted to the UE, although it is possible as described in the PCT application, I. Siomina and M. Kazmi, International application PCT/SE2011/050831, filed on Jun. 23, 2011. In this PCT application, also multi-level patterns have been described where the “level” may be associated with a decision comprising a setting of one or more parameters, the setting characterizing a low-transmission activity, and the parameters being e.g. of any of transmit power, bandwidth, frequency, subset of subcarriers, etc. Such patterns may be associated with either overall transmissions from the node or particular signal(s) (e.g. positioning reference signals, or PRS) or channel(s) (e.g. data channels and/or control channels).
Regarding neighbor cell information, neighbor cell lists (NCLs) are currently specified e.g. for mobility purpose. Transmitting neighbor cell lists from the E-UTRA radio network to the UE is now a standardized feature set forth in 3GPP TS 36.331, Evolved Universal Terrestrial Radio Access (E-UTRA), Radio Resource Control (RRC), Protocol specification, v10.1.0. Transmitting neighbor cell lists is optional in LTE because the UE is required to meet the measurement requirements (e.g. cell search, RSRP and RSRQ accuracy) without receiving an explicit neighbor cell list from the eNodeB. A similar functionally (i.e. signaling of NCL) has been mandatory in E-UTRA since the UE is required to meet more stringent measurement requirements (e.g. cell search, CPICH RSCP and CPICH Ec/No accuracy) only when an explicit neighbor cell list is signaled by the radio network controller (RNC).
The neighbor cell information in E-UTRA may be signaled over RRC either on the Broadcast Control Channel (BCCH) logical channel in a system information block or on the Dedicated Control Channel (DCCH) in an RRC measurement configuration/reconfiguration message.
The neighbor cell related information relevant only for intra-frequency cell re-selection is signaled in the Information Element (IE) SystemInformationBlockType4, whilst IE SystemInformationBlockType5 is used for inter-frequency cell re-selection.
Both system information blocks (SIBs) are signaled over RRC dedicated signaling in the System Information (SI) message through the BCCH logical channel using RLC transparent-mode service. This SI system information and thus the neighbor cell information may be acquired both in RRC_IDLE and RRC-CONNECTED states.
Mapping of SIBs to SI messages is flexibly configurable by scheduling InfoList with restrictions that each SIB is contained only in a single SI message and only SIBs having the same scheduling requirement (periodicity) can be mapped to the same SI message. The transmit periodicity of SIB4 and SIB5 can be configured as one of: 8, 16, 32, 64, 128, 256 and 512 radio frames.
Considering now the contents of the cell information signaled to assist UE mobility in an intra-frequency context, the neighbor cell related information relevant only for intra-frequency cell re-selection is transmitted in the IE SystemInformationBlockType4 and includes cells with specific re-selection parameters as well as blacklisted cells. The maximum number of cells in intra-frequency NCLs or black cell list (BCL) is 16 cells. An NCL contains the Physical Cell Identities (PCIs) and the corresponding cell offset. The offset is used to indicate a cell- or frequency-specific offset to be applied when evaluating candidates for cell re-selection or when evaluating triggering conditions for measurement reporting, and is currently in the range of [−24 dB, 24 dB]. A BCL contains a range of physical cell identities, including the starting (lowest) cell identity in the range and the number of identities in the range. The Physical Cell Identity range is specified in the above mentioned standards document as follows:
PhysCellIdRange ::=SEQUENCE {startPhysCellId,rangeENUMERATED {i. n4, n8, n12, n16, n24, n32, n48, n64, n84,ii. n96, n128, n168, n252, n504, spare2,iii. spare1}OPTIONAL-- Need OP
Considering now the contents of the cell information signaled to assist UE mobility in an inter-frequency context, the neighbor cell related information relevant only for inter-frequency cell re-selection is signaled in the IE SystemInformationBlockType5. The IE includes cell re-selection parameters common for a frequency as well as cell specific re-selection parameters. With the current specification, the parameters that are signalled per carrier frequency and optionally per cell include:
carrier frequency (or ARFCN),
an indicator for the presence of antenna port 1,
allowed measured bandwidth,
reselection parameters accounting for RSRP and
neighbor cell configuration—a bit string of two bits, used to provide the information related to MBSFN and TDD UL/DL configuration of neighbor cells.
The reselection of parameters includes:
selection of an indicator for the required minimum received RSRP in the E-UTRAN cell, in the range of [−140 dBm, −44 dBm],
reselection timer value for E-UTRA indicating the time during which the cell has to be evaluated and ranked, and
reselection thresholds for RSRP when reselecting toward a higher and a lower priority.
The two bit string of the neighbor cell configuration is:
00: not all neighbor cells have the same MBSFN subframe allocation as serving cell,
10: the MBSFN subframe allocations of all neighbor cells are identical to or subsets of that in the serving cell,
01: no MBSFN subframes are present in all neighbor cells, and
11: different UL/DL allocation in neighboring cells for TDD compared to the serving cell.
For TDD, 00, 10 and 01 are only used for same UL/DL allocation in neighboring cells compared to the serving cell.
The optional parameters that may be transmitted with the current specification for inter-frequency NCL, per carrier frequency or per cell, include:
Offset (0 dB default),
the maximum UE transmit power (if absent the UE applies the maximum power according to the UE capability),
speed-dependent scaling factor for the E-UTRA reselection timer value,
the absolute cell reselection priority of the concerned carrier frequency/set of frequencies,
reselection thresholds for RSRP when reselecting towards a higher and a lower priority, and
inter-frequency BCL.
The maximum number of EUTRA carrier frequencies for inter-frequency NCL is 8. The maximum number of cells in inter-frequency NCLs or black cell list (BCL) is 16 cells.
Considering now the requirements applicability for neighbor cell lists signaled in E-UTRA for mobility purpose, as specified in 3GPP TS 36.331, no UE requirements related to the contents of SystemInformationBlock4 or SystemInformationBlock5, which carry intra- and inter-frequency NCI respectively, apply other than those specified elsewhere e.g. within procedures using the concerned system information, and/or within the corresponding field descriptions. This means in E-UTRA the UE is required to meet the measurement requirements without having the NCL. But on the other hand if the NCL is signaled the UE is still required to meet the current measurement requirements since the UE may ignore the NCL or complement it with the blind cell search.
The UE regulatory identifies new cells and maintains a list of certain minimum number of cells for RSRP/RSRQ measurements (e.g. periodic measurements, event-triggered etc). According to 3GPP TS 36.133, with or without blind search as explained above, the UE has to perform measurements for at least a certain minimum number of identified cells. In the RRC_CONNECTED state the measurement period for intra frequency measurements is 200 ms. When no measurement gaps are activated, the UE shall be capable of performing RSRP and RSRQ measurements for 8 identified-intra-frequency cells, and the UE physical layer shall be capable of reporting measurements to higher layers with the measurement period of 200 ms. When measurement gaps are activated the UE shall be capable of performing measurements for at least Ymeasurement intra cells, where Ymeasurement intra is defined in the following equation. If the UE has identified more than Ymeasurement intra cells, the UE shall perform measurements of at least 8 identified intra-frequency cells but the reporting rate of RSRP and RSRQ measurements of cells from UE physical layer to higher layers may be decreased. For FDD:
            Y              measurement        ⁢                                  ⁢        intra              =          Floor      ⁢              {                              X                          basic              ⁢                                                          ⁢              measurement              ⁢                                                          ⁢              FDD                                ·                                    T              Intra                                      T                                                Measurement                  ⁢                                                                          ⁢                  _                  ⁢                                                                          ⁢                  P                  ⁢                                                                          ⁢                  eriod                                ,                Intra                                                    }              ,where Xbasic measurement FDD=8 (cells), TMeasurement_Period, Intra=200 ms is the measurement period for Intra frequency RSRP measurements, Tintra is the time that is available for intra frequency measurements, during the measurement period with an arbitrarily chosen timing. Time is assumed to be available for performing intra frequency measurements whenever the receiver is guaranteed to be active on the intra frequency carrier. For example, when gap pattern #0 is configured and no DRX (Discontinuous Reception) is used or when DRX 40 ms then Tintra=170 ms per 200 ms L1 period because 5 gaps of 6 ms will occur over 200 ms L1 period.
Considering next the signaling of neighbor cell lists to support interference coordination, together with a restricted measurement pattern for neighbor-cell measurements, a list of cells of up to maxCellMeas (32) may optionally be provided. If such a list is provided, then it is interpreted as the list of cells for which the restricted measurement pattern is applied. If the list is not provided, then the UE applies time domain measurement resource restriction for all neighbor cells.
Regarding the requirements applicability for neighbor cell lists signaled in E-UTRA for interference coordination, in the current standard, the same requirements as described above for requirements associated with mobility apply. It has been discussed to mandate signaling the neighbor cell list whenever a restricted measurement pattern is signaled. For cells not in the eICIC list, Rel8/9 mechanisms are claimed to be sufficient. It has been considered that:
a UE is only required to measure and report two restricted cells if the restricted measurement pattern for neighbor cells is configured, and
when a UE is configured for restricted measurements, the UE processing capability of 8 intra-frequency is only required if the cell list is configured together with restricted measurement pattern for neighbor cells.
In the currently standardized environment, numerous problems associated with handling neighbour cell lists, measurements and measurement patterns remain.
One problem is that the UE's behaviour and measurement requirements are ambiguous when cell lists are configured for restricted measurements. For example, when the list is provided together with the pattern configuration, the measurements shall be performed in the restricted subframes, but it is unclear whether the minimum 8 reported cells may also apply exclusively to restricted subframes.
Another problem is that for cells that are not in the list, it is unclear in which subframes the reported measurements have been performed.
In the solutions mandating neighbor cell lists, it is problematic that UE is still required to measure and report on minimum 8 cells in restricted subframes. In this case as above, for cells not in the list, it is unclear in which subframes the reported measurements have been performed.
In other solutions, it is problematic first that the UE may report measurements only for very limited number of cells (e.g. for 2 which is less than the minimum requirements of 8 cells). Secondly if few cells are included in the list, the UE may not report measurements from the remaining cells, thereby degrading the system performance.
Another problem is that there are no measurement requirements for the IDLE state, although the restricted measurement pattern may also be standardized for the IDLE state in the future.
In this document the following abbreviations are used:
3GPP 3rd Generation Partnership Project
BS Base Station
CRS Cell-specific Reference Signal
eICIC enhanced ICIC
eNodeB evolved Node B
E-SMLC Evolved SMLC
ICIC Inter-Cell Interference Coordination
LTE Long-Term Evolution
PCI Physical Cell Identity
RAT Radio Access Technology
RRC Radio Resource Control
SFN System Frame Number
SINR Signal-to-Interference Ratio
UE User Equipment
UMTS Universal Mobile Telecommunications System